ARM: dts: ixp4xx: nslu2: beeper uses PWM

ARM: dts: ixp4xx: nslu2: beeper uses PWM

The beeper in the NSLU2 is just a GPIO connected to a
speaker, so we need to use PWM on the GPIO to get any kind
of sound out.

Tested with some random beeps by enabling INPUT_EVDEV and
running beep.c with e.g. beep 400 for a 400 Hz tone.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/20240627-ixp4xx-dts-v1-1-cdbbe1150873@linaro.org
Link: https://lore.kernel.org/r/20240703-ixp4xx-dts-v1-1-e5149da36f6e@linaro.org
Signed-off-by: Arnd Bergmann <arnd@arndb.de>